1. Field of the Invention
This invention is directed to activated carbon for purifying flue gas, which can be separated magnetically from fly ash and, more specifically, magnetic powdered activated carbon (MPAC) having an enhanced affinity for flue gas constituents such as Hg, the iron on the surface of the carbon catalyzing the oxidation of elemental Hg. The present invention also relates to further enhancing Hg capture by using a photocatalyst (e.g., TiO2, ZnO, SnO2) that may be added to the carbon's surface which when irradiated with UV light creates hydroxyl radicals. The hydroxyl radicals oxidize elemental Hg which adsorbs more readily than elemental Hg.
2. Description of the Related Prior Art
Amongst the numerous hazardous air pollutants (HAPs) currently regulated by the EPA, elemental mercury and mercury containing compounds have recently been highlighted as significant due to their increasing rate of release, and the lack of adequate control technologies. Although the resulting quantity in the environment is usually low, it can transfer to various organisms, and then magnify up the food chain. For example, the concentration of accumulated mercury in some fish can reach levels that are millions of times greater than that in the water. The consumption of such fish by humans, and the resulting buildup of mercury in various tissues may lead to serious neurological and developmental effects such as losses of sensory or cognitive ability, tremors, inability to walk, convulsions, and even death. Methylmercury, the most common form of organic mercury, is almost completely incorporated into the blood stream, and can be transferred through the placenta and into all of the tissues of the fetus, including that of the brain. Because of the health concerns related to eating mercury contaminated fish, bans on fishing in certain regions such as in the Great Lakes have resulted in considerable losses to the economy.
The EPA has estimated that nearly 87% of the anthropogenic mercury emissions are from sources such as waste (as in waste-to-energy facilities) and fossil fuel combustion (as in coal-fired power plants). Recognizing this, control technologies have been employed in an effort to capture and dispose of the mercury found in combustion exhaust gases. Currently, powdered activated carbon (PAC) injection into the flue gas stream is the best available control technology for mercury removal. However, understanding that an estimated 3 kg of activated carbon is needed to remove 1 g of mercury, to meet regulations it is anticipated that PAC injection will cost between $2 and $5 billion annually. Furthermore, PAC's low mercury adsorption efficiency, low applicable temperature range, slow adsorption rate, and lack of adequate regeneration technologies, all have sparked an interest in modifying the material to either decrease costs or improve uptake in hopes for optimization.
Another shortcoming in using PAC injection systems is the accumulation of the waste PAC in the fly ash. Fly ash, the fine particulate fraction of the Coal Combustion Byproducts (CCBs) (i.e., noncombustible inorganics and uncombusted carbon), is collected from flue gas and then commonly sold for the production of concrete and other materials. By using fly ash instead of the lime, cement, or crushed stone materials that are typically used, energy and resources are conserved. However, when the typical fly ash collection devices are coupled with PAC injection systems, the quality of the collected fly ash deteriorates because of the large fraction of carbon in the ash; consequently, revenue generation by selling the fly ash becomes impossible. Current research geared towards separation technologies has yet to find an adequate method to isolate the PAC from the fly ash. Therefore, a method that can easily separate PAC from the fly ash offers the potential to (a) maintain the quality of the fly ash for subsequent use, (b) reuse the PAC, and (c) recover the Hg for various applications.